Ultrahigh-molecular-weight polyethylene (UHMWPE) is synthesized from monomers of ethylene, which are bonded together to form ultrahigh-molecular-weight polyethylene (UHMWPE). These molecules of polyethylene are several orders of magnitude longer than high-density polyethylene due to a synthesis process based on metallocene catalysts. In general, HDPE molecules have between 700 and 1,800 monomer units per molecule, whereas UHMWPE molecules tend to have 100,000 to 250,000 monomers each.
UHMWPE is a linear grade polyethylene, as is high-density polyethylene (HDPE), but it possesses a weight average molecular mass (Mw) of at least 7.5×105 g/mol (according to ASTM D4020). Preferably the UHMWPE has a weight average molecular mass of at least 3×106 g/mol. It has extremely long chains, with molecular weight numbering in millions, usually between 2 and 6 million.
The longer chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. These result in a very tough material, with the highest impact strength of any thermoplastic presently made. UHMWPE, consisting of mainly hydrogen and carbon molecules has an extremely complex structure, which undergoes complex transformations during processing.
Because of the intrinsic properties related to high values of molecular mass, UHMWPE is widely used in several applications which include high abrasion resistance, high-modulus and high-strength tapes and fibers, biaxial films. However, its extremely high melt viscosity severely affects its process-ability while processing it through conventional techniques, such as screw extrusion or injection molding. Problems that are commonly encountered in processing UHMWPEs include die blockage, melt fracture, wall slippage, and a small processing temperature window.
Processing UHMWPE, therefore, requires a proper combination of temperature, pressure, and sufficient time to achieve complete plasticization. Inadequate control of the process can lead to fusion defects stemming from the memory of the powder morphology. Currently, UHMWPE resin is consolidated mainly by compression molding or rams extrusion. Compression molding progresses slowly and involves costly equipment, whereas ram extrusion is relatively cheap. However, some unconsolidated regions in the center of the material may occur. Considerable efforts have been devoted to developing better methods for UHMWPE processing.
Several approaches have been known to improve the process-ability of UHMWPE. For example, solution processing route is followed to reduce the number of entanglement per chain during the manufacture of uniaxial and the biaxial products. Another approach for reducing the number of entanglements is controlled polymerization using single-site catalytic system (Anurag Pandey, Yohan Champoure and Sanjay Rastogi, Macromolecules, 2011, 44 (12), pp 4952-4960).
EP231547 discloses a process for the preparation of polyethylene objects having a high tensile strength and a high modulus by polymerizing ethylene in the presence of a solvent like decalin. Polyethylene produced by the process of EP231547 is only slightly entangled and further disentanglement is easy. The amount of decalin used in the preparations of polyethylene (slightly entangled) for preparing objects like tape, films and the like as disclosed in EP231547 varies between (70-99%).
EP255618 teaches the use of kerosene fraction as a solvent for UHMWPE for preparing a sufficiently disentangled UHMWPE solution. The amount of solvent in such preparations is as high as 90%. An ultrahigh molecular weight polyethylene solution is produced by subjecting a kerosene fraction to UHMWPE which is then cooled to form a gel. UHMWPE is sufficiently disentangled in the solution.
Another method for producing UHMWPE containing compositions with highly beneficial sintering characteristics is disclosed in U.S. Pat. No. 8,063,175. The process includes at least partly disentangling UHMWPE by swelling the UHMWPE with a swelling agent for example decalin and subsequently removing the swelling agent. The swollen polymer comprises about 30% to 99% by weight of swelling agent.
Attempts have also been made to circumvent the intractability of UHMWPE by the addition of solvents, lubricants, plasticizers, and processing aids, as well as polyethylene grades of lower molecular weight as described in U.S. Pat. No. 5,658,992 and U.S. Pat. No. 5,422,061.
Other methods for improving the process-ability of the UHMWPE have been disclosed in U.S. Pat. No. 5,721,334, U.S. Pat. No. 8,003,752, U.S. Pat. No. 7,863,410, U.S. Pat. No. 7,550,555, U.S. Pat. No. 5,621,070, U.S. Pat. No. 5,721,334, U.S. Pat. No. 4,587,163 and U.S. Pat. No. 5,037,928. PCT Publication No. WO2010139720 also discloses a process for preparing disentangled UHMWPE; however it does not deal with the modification of UHMWPE i.e. post polymerization treatment.